A photopolymerizable composition basically contains a photopolymerization initiator and an addition polymerizable compound having two or more ethylenic unsaturated bonds in a molecule (called "a polyfunctional monomer" hereinafter), and changes its tackiness or its solubility in a solvent through the hardening thereof when irradiated with light. Due to these properties, it is used for many purposes, for example, photography, graphic arts, metal surface treatment, ink, and so on. The principles and application examples thereof are described in many books. For instance, there are detailed descriptions in J. Kosar, Light Sensitive Systems, pp. 158-193, J Wiley & Sons, New York (1965), K. I. Jacobson & R. E. Jacobson, Imaging Systems, pp. 181-222, J. Wiley & Sons, New York (1976), and so on.
Recently, image forming systems utilizing light-sensitive microcapsules enclosing a photopolymerizable composition have also been proposed as one of the image forming methods to which photopolymerizable compositions are applied. For instance, methods of forming dye images, which involve exposing imagewise a color forming sheet coated with microcapsules enclosing a dye and photopolymerizable composition comprising a vinyl compound and a photopolymerization initiator, superposing an image receiving sheet on the color forming sheet, and then applying pressure to the whole surface of the superposed sheets, are disclosed in JP-A-57-124343 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-57-179836 and JP-A-57-197538.
In addition, attempts to extend the spectral sensitivities of those photopolymerizable compositions toward the visible region to thereby form digital images using visible laser as a light source or to apply them to a full color photosensitive material have been made in recent years.
For example, methods of spectrally sensitizing photopolymerizable compositions for the purpose of visible laser recording are described in Nippon Shashin Gakkaishi, vol. 49, No. 3, p. 230 (1986) and Kinoh Zairyo (which means "Functional Materials"), the September issue, pp. 48-60 (1983). Further, a method of spectrally sensitizing organic peroxides, functioning as polymerization initiator, by organic dyes is disclosed in JP-A-59-189430. Furthermore, organic boron anion salts of organic cationic dyes are used as novel photopolymerization initiators in EP-Al-223587.
In particular, the method of using the organic boron compound anion salts of organic cationic dye compounds as a light-sensitive initiator has an advantage in that the cationic dye compounds can be selected from a wide variety of dyes, and thereby it becomes feasible to design photopolymerization initiator compositions having sensitivity to intended wavelengths of the visible region.
However, the foregoing method, although successful in giving spectral sensitivities to desired wavelengths of the visible spectrum, failed in inducing satisfactorily high sensitivity.